The invention relates to measuring the angular position of a shaft.
It is desirable in certain applications to determine the angular position of a shaft without direct mechanical connection to it. One such application is in a pressure recording system like that described in U.S. Pat. No. 3,232,115 entitled "Apparatus for Recording Pressure Conditions in Boreholes" by John D. Bennett et al, hereinafter called "the conventional pressure gauge". The conventional pressure gauge may be used to obtain records of variations of pressure in an oil reservoir. The pressure variations coupled with the history of production or other manipulations of the reservoir give valuable information as to conditions in the reservoir.
The pressure sensing element of the pressure gauge is a bellows connected to a Bourdon tube. The Bourdon tube is a flexible tube having a flattened cross section and arranged in the form of a helix. One end of the Bourdon tube is connected to the bellows, which contains a liquid. The other end of the tube is coiled around and coupled to a shaft. When the bellows is subjected to ambient pressure, the liquid transmits the pressure to the interior of the Bourdon tube. The pressure in the tube causes the flattened cross section of the tube to expand toward a circular cross section. The expansion of the tube causes the shaft connected to the free end to rotate. Thus, the rotation of the shaft is a measure of the pressure in the reservoir.
The pressure gauge uses a differential transformer to sense rotation of the shaft and the Bourdon tube. The transformer comprises an oscillator and a detector coil arranged with their axes separated by 90.degree. in order that there will be no coupling between the two. These coils are mounted on the measuring shaft, and a flux coupler, or core piece, is mounted on the measured shaft. As long as the core piece is perfectly aligned with the oscillator coil, there is no coupling of flux between the two coils. This is the null position. When the measured shaft deviates from the null position, the core piece will cause flux coupling between the two coils. The flux coupling will cause a current to flow in the detector coil and associated circuitry, the amplitude and phase indicate the position of the measured shaft with respect to the null position. A motor directed by a control system uses this information to drive the coil assembly back to the null system.
The motor in the control system also connects by reduction gearing to a stylus on a rotatable shaft in a cylindrical recording device. The stylus scribes on a recording surface that is constructed in the form of a cylinder, about the stylus shaft. The stylus shaft is provided with means to allow the stylus to travel up and down the length of the cylindrical recording surface as the shaft turns. Thus, as the shaft turns the stylus will scribe a helical mark on the recording surface.
When the Bourdon element senses a change in pressure the Bourdon shaft will rotate. The control system will follow the movement of the Bourdon shaft and maintain a null, or reference, position. The rotation of the measuring shaft is transmitted via the gearing system greatly magnifies the movement of the Bourdon shaft and thus a small movement in the Bourdon shaft may cause many rotations of the stylus shaft. The pressure change can be computed by counting the number of whole and partial helical scribes on the recording surface. The accuracy of the pressure gauge is increased by causing the movements of the stylus to its recording positions to take place in only one direction. This eliminates a gear backlash resulting from imperfect meshing of the teeth in the gearing mechanism.
Although the pressure gauge provides a novel solution to the problem of sensing pressure with a Bourdon tube, there are several ways in which this operation can be improved. The pressure gauge uses a standard DC type motor that, as is well known in the art, has a small number of bars on the commutator, which limits the number of positions in which the motor rotor may stop. Further there are the drawbacks of limited brush life and brush noise on the power lines. In addition, the differential transformer and its control circuitry consumes a relatively large amount of power.